Both of my tensioners were replaced at 150,000 miles and have been okay since then. I just inspect them each time the belts are serviced.

2002 X5 3.0 306,800 miles
2014 428i 21,000 miles

2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles

1970 Firebird - Under restoration